Process of selective fluorine removal from zinc concentrates and manganese dioxide



United States Patent PROCESS OF SELECTIVE FLUORINE REMOVAL FROM ZINC CONCENTRATES AND MAN GA- NESE DIOXIDE Bruno Orlandini and WalterSchmittroth, Kellogg, Idaho, assignors, by mesne assignments, to TheBunker Hill Company, Kellogg, Idaho, a corporation of Delaware NoDrawing. Filed Sept. 25, 1967, Ser. No. 670,441

Int. Cl. C22d 1/22 U.S. Cl. 204119 7 Claims ABSTRACT OF THE DISCLOSUREDESCRIPTION OF THE INVENTION This invention relates to the electrolyticprocess for recovering zinc and more particularly to processes forpreventing fluorine from etching the aluminum cathodes as the zinc isbeing deposited thereon in the electrolytic bath.

To properly understand and appreciate this invention it is necessary tobe familiar with the general electrolytic process for recoveringmetallic zinc having a purity of greater than 99.99% from zinc ore.

The zinc is generally mined from the earth as a zinc sulfide. The zincsulfide ore is often referred to as zinc blend. After crushing, the oreis concentrated by flotation. A host of other materials of variousconcentrations besides zinc sulfide is included in the concentratedepending upon the source of the ore. Iron compounds are generallyincluded in the concentrate. Often the concentrate contains fluorinecompounds in concentration of more than 3 milligrams per hundred gramsof concentrate. Although the zinc sulfide is somewhat soluble inrelatively strong acid, it has been found that by roasting theconcentrate, the zinc sulfide is transferred into a more solublecompound of zinc oxide. The roasted zinc concentrate is referred to aszinc calcine. The calcine is then leached in a sulfuric acid solution toseparate those impurities that are soluble in the electrolyte from thosematerials that are insoluble in the electrolyte. Zinc oxide and someimpurities dissolve in the sulfuric acid leach. Included in the solubleimpurities are generally small amounts of metallic compounds of iron,cobalt, arsenic, antimony, nickel, etc. Fluorine compounds such assodium fluoride and calcium fluoride also dissolve in the leach.

If the amounts of the dissolved metallic impurities are greater thandesired, various additives are added to precipitate the metallicimpurities from the solution before the solution passes to theelectrolytic cell. Generally manganese dioxide is added to oxidize theiron to the ferric state which in turn precipitates out as ferriccompounds. Frequently the manganese dioxide additive contains fluorinecompounds. The leach pulp is then filtered to remove the precipitate.Metallic zinc is then added to the leach filtrate to precipitate many ofthe metallic impurities such as copper, cobalt, nickel, etc. The leachfiltrate is again filtered to remove the precipitates. The concentratedzinc solution is then carried to the electrolytic cell Where anelectrical force is applied between the cathode and anode causing thezinc ions to migrate and deposit upon a cathode.

The cathodes are generally made of metallic aluminum wlth a coating ofaluminum oxide formed on the surface thereof to act as a barrier toprevent the zinc from bondmg to the metallic aluminum. However, if theelectrolyte contains fluorine, the fluorine etches away the aluminumoxide protective barrier enabling the zinc to bond directly to themetallic aluminum. Such a metallic bond makes it almost impossible tophysically remove (strip) the zinc from the cathode without damaging thecathode or redissolving the zinc. As a practical matter, if a metal tometal bond occurs, substantial additional costs are incurred to removethe zinc from the cathode.

Several attempts have been made to minimize the fluorine etching of thecathodes. One method used, is to bleed the electrolyte from theelectrolytic cell when the fluorine concentration in the electrolyteexceeds a certain value. The electrolyte bled from the cell contains asubstantial amount of valuable zinc and acid which represents lostproduction or requires costly further processing to remove the fluorineand recycling the zinc solution back to the electrolytic cell.

Another method used to minimize the fluorine etching of the cathodewhile the zinc is being deposited on the cathode is to let the zincsolution sit in the cell with the cathode immersed therein for about 15minutes before the electrical force is applied between cathode andanode. During this interval the surface of the aluminum oxidizes to forma reinforced protective barrier. Such amethod substantially reduces theefliciency of the electrolytic process, because 15 minutes of each bathcycle is unproductive and may be considered down time necessitatingadditional labor to energize the cell.

One of the principal objects of this invention is to provide a vastlyimproved method of preventing the fluorine etching of the cathode.

An additional object of this invention is to remove the fluorinecompounds that are soluble in the electrolyte before the constituentsare added to the electrolytic cell.

A further object of this invention is to provide a preleaching processfor removing electrolyte soluble fluorine compounds from the zincconcentrate and the manganese dioxide before they are added to theelectrolytic cell.

An additional object of this invention is to pre-leach the electrolyticsoluble fluorine compounds from the zinc concentrate before the zincconcentrate is roasted.

This invention involves the removal of the electrolyte soluble fluorinecompounds from the principal sourcesthe zinc concentrate and themanganese dioxide additive.

With respect to the zinc concentrate, the electrolyte soluble fluorinecompounds are removed therefrom prior to roasting. It has been foundthat by contacting the zinc concentrate (zinc sulfide) with a weaksulfuric acid solution (pre-leaching) that the electrolyte solublefluorine compounds will dissolve without dissolving appreciable amountsof the zinc sulfide. It has been found that a weak sulfuric acid leachsolution of between 1% and 5% sulfuric acid by weight is particularlyelfective in dissolving the soluble fluorine compounds Withoutdissolving an appreciable amount of zinc sulfide.

In performing the pre-leaching process the following preferred procedurewas found to be particularly effective. The zinc concentrate was mixedwith water to form a slurry in the proportions of grams of zincconcentrate per 250 milliliters of slurry. The 250 milliliters of slurrywas mixed with an equal amount of a weak acid solution. The 'weak acidsolution was formulated by adding between 20 and 100 grams of sulfuricacid per liter of water. The weak acid solution and the slurry wereagitated to increase the rate of dissolution of the soluble fluorinecompounds.

3 At temperatures between 2040 C. it has been found that four hours ofagitation was sufiicient.

After the leach pulp is agitated the solution containing the dissolvedfluorine is filtered from the zinc concentrate. Utilizing this procedureit was found that the soluble fluorine concentration in the zincconcentrate could be readily and effectively reduced well below 3milligrams per 100 grams of zinc concentrate. The zinc concentration isthen roasted to form the zinc oxide. In this manner the electrolytesoluble fluorine compounds are removed from the zinc concentrate toprevent the soluble fluorine compounds from being carried to theelectrolytic cell.

This pre-leaching process has been utilized on zinc concentrate batchesfrom many parts of the world. The pre-leaching of zinc concentrate Awith a 1% sulfuric acid solution removed 21.2 milligrams of fluorine and0.31 gram of zinc per 100 grams of zinc concentrate. Ntilizing asulfuric acid solution, 24.1 milligrams of fluorine and 1.06 grams ofzinc per 100 grams of cencentrate were pre-leached from the zincconcentrate to reduce the fluorine content well below the maximumallowable of 3 milligrams of fluorine per 100 grams of zinc concentrate.

The pre-leaching of zinc concentrate B with a 1% sulfuric acid solutionremoved 7.6 milligrams of fluorine and 0.20 gram of zinc per 100 gramsof zinc concentrate. Utilizing a 5% pre-leaching sulfuric acid solutionwith the zinc concentrate B, 25.0 milligrams of fluorine and 0.64 gramof zinc per 100 grams of zinc concentrate were removed reducing thefluorine concentration well below the maximum allowable. With respect tozinc concentrate C which came from an area having a very largeconcentration of acid soluble fluorine compounds in the ore, 353milligrams of fluorine and 1.06 grams of zinc were removed from theconcentrate utilizing a 5% weak acid leach solution.

In like manner the electrolyte soluble fluorine compounds may bepre-leached from the manganese dioxide additive so that soluble fluorinecompounds do not enter the electrolyte when the manganese dioxide isadded during the leaching step of the process.

It should be understood that various other preferred embodiments mayreadily be devised by those skilled in the art that incorporate theprinciples of the invention and fall within the scope thereof.Therefore, only the following claims are intended to define thisinvention.

What is claimed is:

1. In a process for recovering metallic zinc from ore containingfluorine compounds by separating the ore to form a zinc concentrate,roasting the zinc concentrate to form zinc calcine, dissolving thecalcine in a sulfuric acid solution to form a zinc electrolyte andseparating the zinc from the electrolyte by electrolysis utilizing acathode having an aluminum oxide coating that is susceptible to fluorideetching, the improvement in which the process includes a pre-leachingsub-process of removing the aqueous or acid soluble fluorine compoundsfrom the zinc concentrate prior to roasting by contacting the zincconcentrate with a weak acid leach solution having a sulfuric acidconcentration which is suflicient to dissolve the soluble fluorinecompounds without dissolving appreciable amounts of the zincconcentrate.

2. The process as defined in claim 1 wherein the preleaching sub-processis performed with a leach solution containing between 1% and 5% sulfuricacid by weight.

3. The process as defined in claim 1 wherein the preleaching sub-processincludes the addition of water to the zinc concentrate to form a slurryprior to mixing with the weak acid solution.

4. The process as defined in claim 3 wherein the water and zincconcentrate are mixed to form a slurry in proportion having grams ofzinc concentrate per 250 milliliter and wherein the slurry is mixed withan equal amount of weak acid solution having between 20 and 100 grams ofsulfuric acid per liter of solution.

5. The process as defined in claim 4 wherein the resultant slurry acidsolution is agitated to increase the dissolution of the soluble fluorinecompounds.

6. In a process for recovering metallic zinc from ores containing ironcompounds by separating the ore to form a zinc concentrate, roasting thezinc concentrate to form zinc calcine, dissolving the calcine in asulfuric acid solution to form a zinc leach pulp, adding manganesedioxide to the leach pulp to oxidize and precipitate the iron,separating the precipitate from the pulp to form a zinc solution andseparating the zinc from the solution by electrolysis utilizing acathode having an aluminum oxide coating that is susceptible to fluorineetching, the improvement in which impure manganese dioxide is preleachedprior to adding to the leach pulp to remove solu ble fluorine compoundsfrom the manganese dioxide to prevent the fluorine etching of thecathode, said preleaching being performed by contacting the impuremanganese dioxide with a weak acid leach solution that has a sulfuricacid concentration which is sufiicient to dissolve the soluble fluorinecompounds without dissolving appreciable amounts of manganese dioxide.

7. The process as defined in claim 6 wherein the manganese dioxideconcentrate is pre-leached with a leach solution containing between 1%and 5% sulfuric acid by weight.

References Cited UNITED STATES PATENTS 6/1925 Greenawalt 204119 7/1899Hoepfner 204118 U.S. Cl. X.R. 204-118

